Wireless communication systems are well known in the art. Generally, such systems comprise communication stations which transmit and receive wireless communication signals between each other. Typically, base stations are provided which are capable of conducting wireless concurrent communications with a plurality of subscriber stations generically known as wireless transmit/receive units (WTRUs), which include mobile units. Generally, the term base station includes, but is not limited to, a base station, Node-B, site controller, access point, or other interfacing devices in a wireless environment. The term WTRU includes, but is not limited to, a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment.
In Universal Mobile Telecommunications Systems (UMTS) as specified by the Third Generation Partnership Project (3GPP), base stations are called Node Bs, subscriber stations are called User Equipments (UEs) and the wireless CDMA (Code Division Multiple Access) interface between the Node Bs and UEs is known as the Uu interface.
A typical UMTS system architecture in accordance with current 3GPP specifications is depicted in FIG. 1a. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an interface known as Iu which is defined in detail in the current publicly available 3GPP specification documents.
The UTRAN is configured to provide wireless telecommunication services to users through UEs via the Uu radio interface. The UTRAN has base stations, Node Bs, which collectively provide for the geographic coverage for wireless communications with UEs. In the UTRAN, groups of one or more Node Bs are connected to a Radio Network Controller (RNC) via an interface known as Iub in 3GPP. The UTRAN may have several groups of Node Bs connected to different RNCs. Two RNCs are shown in the example depicted in FIG. 1a. Where more than one RNC is provided in a UTRAN, inter-RNC communication is performed via an Iur interface.
In existing systems, when a mobile unit is first turned on or traverses into a region of multiple base station coverage, a determination must be made as to which base station the mobile unit will be paired for conducting wireless communication. Depending upon system design, the mobile unit, the communications network or the base stations will determine the pairing between each mobile unit and a base station.
In one type of configuration, a mobile unit monitors common signals from all base stations that it receives and synchronizes itself to the base station with the best quality of service signal (QoS). In such systems, a beacon signal radiated by each base station is an omnidirectional high powered transmission that has a tendency to generate interference.
Smart antennas that include beamforming capability are widely regarded as a promising technology for enhancing capacity and/or coverage of wireless radio access systems, such as 3GPP mobile communications systems. The distinguishing feature of a wireless radio access system employing smart antennas is that a user may be spatially isolated. Radio transmissions directed toward, or received from, a user are isolated in such a way that minimizes interference to or from other users. FIG. 1b illustrates a smart antenna of a Node B focused at a UE of a 3GPP system.
Wireless radio access systems, such as UMTSs that employ smart antennas, derive two-fold system-level benefits by using highly focused directional antennas. First, the system capacity improves as a result of the reduction in generated interference. Second, the system coverage improves, resulting in an enhanced link budget. The increase in radio coverage from the use of smart antenna technology represents a particularly attractive feature for wireless communications systems. The application of smart antenna technology, including beamforming, is rather straightforward once a radio link is established between a mobile and a radio access point to exchange information over a dedicated channel.
In addition to dedicated radio links, common channels are typically employed in wireless radio access systems. Common channels are established for various purposes, such as: (1) allowing for the temporal or frequency synchronization of mobiles, for example, a 3GPP shared synchronization channel (SCH); (2) broadcasting of system information that is essential for registration to the network upon power-up, for example on a 3GPP broadcast channel (BCH); and (3) paging of idle-mode mobiles, for example on a 3GPP paging indicator channel (PICH), paging channel (PCH), and forward access channel (FACH).
In a statistical sense, the geographical coverage that is provided by downlink common channels defines the coverage area of a base station, which in UMTS, is commonly referred to as a cell. More specifically, the service area provided by a wireless radio access system is determined from the coverage of common channels.
A significant increase in cell area covered by a wireless radio access system using smart antenna technology is enabled by employing highly directional antennas that boost the gain of such systems. Directional antenna gain is achievable when the position of an antenna may be estimated by its peer antenna, and vice versa. Such circumstances are generally fulfilled when a dedicated radio link is established between a mobile and a radio access point.
The usage of smart antennas for the transmission and reception of common channels is not defined in wireless radio access systems existing 3GPP specifications and the advantages resulting from the use of smart antenna technology have yet to be exploited for the transmission and reception of common channels. A reason for this is that coverage of common channels, such as BCH and PICH, must be guaranteed for all mobiles, including those for which the location is unknown. More specifically, a radio access network must ensure that all mobiles may reliably synchronize with the network, read broadcast information, and monitor pages, to name a few. This complication results in wireless radio access systems that transmit common channels using conventional omnidirectional antennas that cover entire cells or cell sectors.
In order to match the extended coverage of dedicated channels using smart antennas, the transmission power of downlink common channels may be increased. However, an increase in transmission power by all radio access points, for example, base stations, also results in an increase in interference. Such a solution is ineffective in wireless radio access systems that are limited by interference. The present preferred solution takes advantage of smart antenna technology to extend coverage while minimizing interference.